This is because of the large variety of the features being searched in the images, such as people faces, cars, etc. On the contrary, traffic signs a) are made with vivid and specific colors so as to attract the driver’s attention and to be distinguished from the environment b) are of specific geometrical shapes (triangle, rectangle, circle - ellipse) and c) for each sign there is a specific template. It is therefore rather easy to develop an algorithm in such a way that the computer has “a priori knowledge” of the objects being searched in the image.